The present invention relates to an air-fuel injection system for a turbojet engine, more particular such a system having an improved air swirler.
Present day turbojet engines are required to generate a high performance level, while at the same time maintaining a low pollution rate. These contradictory parameters have required tradeoffs to be made between full power operational characteristics to minimize smoke emission and maximize the life of the engine components, and low speed operational characteristics such as flame stability and engine efficiency. As the pollution standards become increasingly more strict, the combustion chambers' size and the requirement to utilize a diversity of fuels renders the aforementioned characteristics increasingly difficult to obtain.
Two module combustion chambers are known wherein one of the chambers is intended for low speed operation and the other for full operation. However, these systems increase the bulk and the weight of the engine and, therefore, are not a complete solution to the problem.
It is also known to use variable geometry injection systems wherein diaphragms or flaps control the air intakes to the combustion chambers. This allows a substantial reduction in the combustion volume and, consequently, the bulk of the chamber.
It is known to utilize this type of variable geometry injection systems with aerodynamic injectors having intermediate bowl-shaped members. In this system, the fuel injectors are mounted in the upstream end of the combustion chambers with the intermediate bowl-shaped member interposed between the fuel injector and the combustion chamber. The bowl-shaped member typically comprises a shroud which flares outwardly in the downstream direction and is provided with a plurality of small diameter holes to allow air to enter the atomized fuel cone emanating from the injector. The bowl shaped member produces turbulent air flow to improve the fuel atomization in the air-fuel mixture.
In order to further improve the performance characteristics of these aerodynamic injectors with intermediate bowl-shaped members, the outer swirlers as well as the air intakes of the bowl openings may be equipped with a diaphragm to control the flow of air through these elements in order to match the air-fuel mixture richness at the bowl exhaust to all operating conditions.
U.K. Pat. No. 2,085,147 discloses a typical example of this type of swirler system in which a control diagrham in the form of a thin cylindrical shell surrounds an annular flange defining air intake orifices. Neither the control diaghragm nor the air intake orifices have vane systems therein to guide and deflect the incoming atomizing air. In this particular system, the air guidance is achieved only by means of curved extensions on the outside of the control diaghragm.
U.S. Pat. No. 4,534,166 to Kelm et al. show an axial-type swirler utilized in a combustion chamber structure without having an intermediate bowl shaped member. The slopes of the diaghragm vanes differ from those of the swirler structure such that the air-intake angle can be varied in relation to the operating modes of the engine. However, this system suffers from the drawback that the external swirler is located at the very upstream end of the combustion chamber and substantial wakes or turbulence are presented in the fuel cone due to the location of the swirler vanes. The wakes degrade the air-fuel mixture resulting in less efficient combustion chamber operation. The system shown in Kelm et al. does not permit attenuation of these wakes.